Export of internal Abseil changes

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552959395a367b202e00fc7680e3500e4d15a341 by Abseil Team <absl-team@google.com>:

In span_test, correctly describe matches with negation, and remove redundant "is".

negation is true when the match is negative.

PiperOrigin-RevId: 267064695
GitOrigin-RevId: 552959395a367b202e00fc7680e3500e4d15a341
Change-Id: Ic876f30170e7e0008c13fdc6269462cfc5737e0f